1. Field of the Invention
The present invention relates to an oxide having a perovskite structure, barium titanate, and a manufacturing method therefor, dielectric ceramic, and ceramic electronic component.
2. Description of the Related Art
An oxide having a perovskite structure, for example, barium titanate, has been used as a dielectric material for electronic components such as capacitors. Recently, it is required that the oxide having the perovskite structure be a powder having a smaller particle diameter.
For example, miniaturization and an increase in capacitance in a monolithic ceramic capacitor have been attempted by decreasing the thickness of the dielectric ceramic layer, and for such a purpose, oxides having the perovskite structure, for example, barium titanate having not only higher dielectric constant, but also smaller particle diameter, have been required. Hitherto, barium titanate having a particle diameter of about 0.2 xcexcm or less has been produced by wet methods such as hydrolysis and hydrothermal synthesis.
In barium titanate synthesized by a conventional wet method, however, there are problems in that about 0.2 to 3% by weight of OH groups remain in the particle, and in spite of small particle diameter, the crystal system is a cubic system or a tetragonal system in which the ratio of the a axis to c axis in the crystal lattice, that is, the c/a axial ratio (x), is so small as not to fulfill the relationship represented by the formula xxe2x89xa71+(kxe2x88x921)xc3x970.3, where k indicates a theoretical value of the c/a axial ratio (in barium titanate, k=1.011 and xxe2x89xa71.0033), and therefore sufficient ferroelectricity for a capacitor material is not exhibited.
In order to solve the aforementioned problems, it has been attempted to produce barium titanate having a large c/a axial ratio and exhibiting sufficient ferroelectricity by reheating the synthesized barium titanate for removal of the OH groups. Since cavities are, however, generated at places where OH groups were present and the growth of particles of barium titanate occurs during the reheating, it becomes difficult to produce barium titanate having a particle diameter of as small as about 0.2 xcexcm or less. At the same time, when the thickness of dielectric ceramic layer in laminated ceramic electronic components using such a barium titanate, for example, monolithic ceramic capacitors, is decreased to several xcexcm or less, it also encounters problems of short circuit occurrence and degradation in other characteristics.
Accordingly, the object of the present invention are to provide a manufacturing method for an oxide having the perovskite structure, for example, barium titanate, having a small particle diameter, having small amounts of OH groups, having a large c/a axial ratio and exhibiting sufficient ferroelectricity and to provide the oxide having the perovskite structure produced by the aforementioned manufacturing method, a dielectric ceramic produced using a powder made of such an oxide and a laminated ceramic electronic component produced using such a dielectric ceramic.
A manufacturing method for an oxide having a perovskite structure according to the present invention comprises steps of preparing a metallic oxide powder having a specific surface area of about 10 m2/g or more and preparing a metallic carbonate powder, mixing the metallic oxide powder and the metallic carbonate powder, and heat-treating the resulting mixed powder at a partial pressure of oxygen of about 2xc3x97103 Pa or less.
At least two kinds of metallic oxide powders and metallic carbonate powders may be used, respectively.
The present invention also relates to an oxide having the perovskite structure. This oxide is preferably a powder having a particle diameter of about 0.03 to 0.2 xcexcm and the amount of OH groups in the crystal lattice may be less than about 0.2% by weight.
The measured value of the c/a axial ratio in the crystal lattice of this oxide indicated by x and the theoretical value of the c/a axial ratio indicated by k of this oxide are preferably in a relationship fulfilling the condition represented by the formula xxe2x89xa71+(kxe2x88x921)xc3x970.3.
The aforementioned oxide having the perovskite structure can be synthesized with advantage by, for example, the aforementioned manufacturing method.
The manufacturing method for the oxide having the perovskite structure according to the present invention can be specifically applied to a manufacturing method for barium titanate.
The specific manufacturing method for barium titanate is composed of steps of preparing a TiO2 powder having a specific surface area of about 10 m2/g or more and preparing a BaCO3 powder, mixing the TiO2 powder and the BaCO3 powder, and heat-treating the resulting mixed powder.
In the case in which the aforementioned step of heat-treating is performed at a partial pressure of oxygen of about 2xc3x97103 Pa or less and exceeding about 2xc3x97102 Pa, a heat-treating temperature within the range of about 700 to 1,100xc2x0 C. may be applied. In the case in which the aforementioned heat-treating is performed at a partial pressure of oxygen of about 2xc3x97102 Pa or less, a heat-treating temperature within the range of about 600 to 1,100xc2x0 C. may be applied.
The present invention also relates to barium titanate. This barium titanate is preferably a powder having a particle diameter of about 0.03 to 0.2 xcexcm and the amount of OH groups in a crystal lattice is less than about 0.2% by weight.
The c/a axial ratio of this barium titanate is preferably 1.0033 or more.
The aforementioned barium titanate can be synthesized with advantage by, for example, the aforementioned manufacturing method.
The present invention also relates to a dielectric ceramic produced by firing the aforementioned powder made of the oxide having the perovskite structure such as barium titanate.
The present invention also relates to a ceramic electronic component including the aforementioned dielectric ceramic and electrodes facing to each other with the dielectric ceramic therebetween.
The present invention also relates to a laminated ceramic electronic component including a laminate composed of a plurality of laminated dielectric ceramic layers and an internal conductor formed along a specified interface between the dielectric ceramic layers. Herein, the aforementioned dielectric ceramic layers are made of a dielectric ceramic according to the present invention.
In the laminated ceramic electronic component according to the present invention, the dielectric ceramic layer may be made of a dielectric ceramic produced by firing a powder, having a particle diameter of about 0.03 to 0.1 xcexcm, made of an oxide having the perovskite structure, e.g., barium titanate, and the thickness of the dielectric ceramic layer is preferably about 1 xcexcm or less.
In the aforementioned laminated ceramic electronic component according to a specific embodiment, a plurality of external electrodes arranged at positions different from each other on the end face of the aforementioned laminate are further provided and a plurality of internal conductors are formed with an end edge thereof being exposed on the end face of the laminate so as to be electrically connected to one of the external electrodes. The configuration of this laminated ceramic electronic component may be applied to, for example, monolithic ceramic capacitors.